Exploring the tunnels in Gaza

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A series of fragile cease-fires has replaced the month-long battle between Israel and Hamas. If a more permanent truce now seems within reach, that is partly because Israel has achieved one of its major strategic objectives. It controls the network of underground tunnels that Hamas has been using to stage surprise attacks on military personnel inside Israel.

From the beginning, these tunnels have been at the center of the renewed violence in Gaza. But what kind of tunnels are we talking about?

If you’re imagining narrow passages of hollowed-out earth, think again. These are deep, concrete structures that took years to build. They are both difficult to detect and difficult to destroy.

What do these tunnels look like? And how many are there?

The tunnels go as deep as 75 feet below the surface. From there, they can extend for upwards of one mile and are often wide enough for people to move past each other in opposite directions. They carry electricity and include adjoining rooms. And for concealment, entrances and exits tend to be located inside homes and other structures.

The Israelis claim to have located over 30 tunnels since the fighting began. To date, roughly half of these have been destroyed, and the rest have been effectively neutralized.

How did Hamas build these tunnels?

Hamas fighters seized control of the Gaza Strip in 2007 after a struggle with Fatah, the other major Palestinian faction. At that time, Egypt -- which also shares a boundary with Gaza -- sealed its side of the border, partly to show support for Fatah but also to prevent violence from spilling into its territory.

The closing of the Gaza-Egypt border severely restricted the movement of people and goods. Hamas responded by building a series of smuggling tunnels to bring goods into Gaza from the Egyptian side. By one estimate, roughly $700 million of materials moves through the tunnels each year, which would amount to nearly 10 percent of the Gaza economy.

The expertise Hamas developed building and maintaining these smuggling tunnels helped them construct the offensive tunnels into Israel.

When did Israel learn about the tunnels

Although Israel seems to have underestimated the size and sophistication of the offensive tunnels that now run under its border, it has known about the existence of some of these tunnels since 2006. That’s the year an Israeli soldier named Gilad Shalit was captured by Hamas militants who had entered Israel through an underground tunnel.

Shalit was eventually returned to Israel as part of a prisoner exchange in 2011. Destroying the tunnels didn’t become a top military priority until the recent fighting, when their full scope and tactical value became clear.

Has Hamas used the tunnels for other attacks?

Over the course of the last month, Hamas fighters have used the tunnels to stage a series of raids against Israeli military outposts and personnel, killing roughly 10 Israeli soldiers.

To date, the tunnels have not been used in terrorist attacks against civilians, but that remains a pressing concern among Israelis.

Are the tunnels easy to destroy?

Not at all. Egypt has tried repeatedly to destroy the less-sophisticated smuggling tunnels running under its border, without much success. In 2013, it even tried flooding the tunnels with raw sewage.

The offensive tunnels that cross into Israel are too deep to detect or destroy from the air. To tragger a collapse, they have to be mined with explosives. Sometimes this is done by having Israeli soldiers and engineers go through the tunnels and place charges. In other cases, holes can be drilled from the surface and explosives lowered into place, which helps limit risks and avoid boobytraps.

Even if they can be destroyed, though, there’s a question about whether Hamas will rebuild the tunnels as soon as Israeli troops depart. To prevent that, Israel is considering a variety of approaches, including an underground system of fiber-optic cables and microphones.

What happens next?

For years now, Israel and Hamas have been playing a kind of cat and mouse game. When Israel built a wall to stop suicide bombers from entering Israel, Hamas increased its investment in rocket technology. As Israel’s “Iron Dome” anti-missile system got more effective, Hamas went underground and built a robust tunnel network. If Israel now succeeds in cutting off all tunnel attacks, Hamas may shift its tactics yet again.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the U.S. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz.